Members of the contact lens industry recognize that, on occasion, a conventional clear, colorless contact lens may include a yellow tint. The yellow tint can adversely affect consumer acceptance of the contact lens. The yellow tint can result from improper processing of the contact lens polymer, or from certain ingredients in the lens polymer. Therefore, it is sometimes difficult to predictably avoid the yellow tint in the manufacture of clear, colorless lenses.
Further, due to public concern over the potentially harmful effects of ultraviolet radiation (UV), some contact lens manufacturers have begun to add UV-absorbing compounds to contact lenses. Unfortunately, compounds that absorb UV radiation also tend to impart an undesirable yellow tint to the lenses, if the compounds are present in sufficient amounts to absorb strongly above 380 nm. Therefore, what was once an unpredictable problem may become an unavoidable problem for clear, colorless lenses that contain or are treated with a UV-absorber.
For many clear lenses, the yellow tint is not particularly troublesome because manufacturers already add a handling tint to these lenses. A handling tint is a color (usually green or blue) that is dispersed evenly throughout the lens or bonded to the lens surface in sufficient amounts to make the contact lens more easily observable for handling purposes. Typically, the amount of handling tint added will not substantially modify the wearer's eye color or affect the passage of light through the lens.
In the absence of the regular use of a handling tint, a prudent manufacturer would, nevertheless, mask a clear, UV-absorbing lens with a more desirable masking tint to hide the undesirable yellow tint imparted by the UV-absorbing compound. Handling tint and masking tints are generally made from the same or similar colorants, and in the same or similar concentrations. Therefore, for the purposes of the present disclosure, they are referred to collectively as masking tints. In addition, clear lenses that contain such traditional masking tints are referred to as tinted, clear lenses.
However, even the use of a traditional masking tint may not adequately mask the yellow tint for a clear lens. For a thicker lens, such as a toric lens, the traditional masking tint may not adequately hide the yellow color. In this case, adding enough masking tint to hide the yellow tint may also result in a substantial effect on the wearer's apparent eye color. Further, many clear lens consumers also prefer a colorless lens. For these consumers, the absence of a traditional handling tint will only highlight any yellow tint imparted by an UV-absorber or by other sources.
Moreover, in the context of a lens that is used to change the wearer's eye color (a colored lens), the yellow tint poses a substantial problem. In a colored lens, either a section of the lens or the whole lens contains sufficient color to modify a wearer's eye color (hereinafter "modifying color"). As an example, one can use the lens patented by Knapp in U.S. Pat. No. 4,582,402, which has achieved considerable consumer acceptance. In the Knapp lens, the modifying color is printed onto the contact lens as a pattern of color elements (generally dots) over the region of the lens that substantially covers the iris. However, the interstices between the color elements, and the other sections of the lens (e.g., those that cover the pupil and the sclera) remain essentially clear and colorless (occasionally, a barely visible pattern or a small pattern is printed on part of the sclera region).
For such a lens, the presence of a yellow tint, whether or not due to the addition of a UV-absorber, dramatically reduces its acceptance by consumers. Efforts to overcome the yellow tint by adding a traditional masking tint did not achieve satisfactory results because traditional masking tints tend to unacceptably alter the modifying color. In addition, efforts to mask the yellow tint in colored lenses did not predictably alter the appearance of the lens. For example, a colored lens containing both an UV-absorber and a masking tint will modify the lens appearance in a different way than the same lens that does not contain the UV-absorber.
Blue or green are the traditional colors for masking tints because they have achieved general acceptance by consumers. Although violet pigments have been used in other contexts for contact lens (e.g., as a modifying color or part of a modifying color pattern, and for the small, barely visible printing on the sclera of a color lens), violet has not been used as a masking tint. By itself, and at higher concentrations, a violet masking tint tends to impart a darker appearance to the contact lens. At even higher concentrations, the violet color will adversely modify the wearer's eye color. Contact lens manufacturers do not believe a contact lens that contains such concentrations of violet tint can achieve consumer acceptance.
Nevertheless, it would be desirable, especially in the context of adding an UV-absorbing compound, to formulate a lens wherein the presence of yellow tint can be effectively masked. It would be especially desirable to mask the yellow tint in a colored lens without adversely altering the modifying colors.